1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer using an electrophotographic method, and a control method for the image forming apparatus, and in particular to an image forming apparatus that adopts image density control using density sensors.
2. Description of the Related Art
Conventionally, image forming apparatuses such as copiers and printers using an electrophotographic method have adopted image density control using a density sensor so as to maintain image densities of continuously output prints uniform. Examples of the density sensor include an optical sensor comprised of a light-emitting unit consisting of an infrared light-emitting device such as an LED, and a light-receiving unit consisting of a photodiode.
In an example of the image density control, unfixed toner patches formed on an intermediate transfer unit with predetermined timing are read by the density sensor located in the vicinity of the intermediate transfer unit. Then, toner patch densities equivalent to amounts of toner are measured from detected values (output values) from the density sensor, and print data is corrected based on the toner patch densities. In another example of the image density control, a development contrast is corrected based on toner patch densities obtained by the density sensor. In the case of the image density control in which the development contrast is corrected, a dark potential VD on a photosensitive drum and a direct-current voltage VDC applied to a development sleeve are corrected, or laser power is corrected.
Thus, in the image density control, various image forming conditions are usually given feedback based on toner patch densities obtained by the density sensor. For this reason, the accuracy with which toner patch densities are measured by the density sensor greatly affects stability in image densities of prints.
On the other hand, examples of techniques required to measure toner patch densities using the density sensor include base correction. Aging variations in the surface gloss of a base on which toner is put (for example, an intermediate transfer unit 27 shown in FIG. 1) affects output values from the density sensor when toner patch densities are measured. Thus, with consideration given to aging variations in the surface gloss of the base, base correction is performed to cancel the effects on toner patch densities (sensor output values).
FIG. 12A is a diagram showing the relationship between the surface gloss of the base and the number of prints made by a printer. In the figure, the horizontal axis represents the number of prints made by the printer, and the vertical axis represents the average surface gloss of the base over one circumference thereof. As shown in the figure, the surface gloss of the base varies over time with the number of prints. This is considered to be because the rotating base is ground by toner, external additives, carriers, and the like remaining on a cleaning blade in a cleaning mechanism that picks up toner remaining on the base.
FIG. 12B is a diagram showing the relationship between the output from the density sensor and the number of prints made by a printer. In the figure, the horizontal axis represents the number of prints made by the printer, and the vertical axis represents the sensor output from the density sensor. As shown in the figure, sensor output values obtained by the density sensor reading toner patches with the same density formed on the base vary with the number of prints.
As will be understood from FIGS. 12A and 12B, aging variations in the surface gloss of the base affect sensor output values obtained by the density sensor reading toner patches with the same density formed on the base. The effects on the sensor output values can be cancelled using a base correction method described below.
Assume that an average value of sensor output values obtained by reading a surface of the base over one circumference (which means, for example, one circumference of an intermediate transfer unit) is R (base one-circumference average), and a sensor output value obtained by reading a toner patch is R (toner patch). In this case, a density DENS of a toner patch is calculated according to (Equation 01) given below. As a result, aging variations in sensor output values, which are obtained by reading toner patches with the same density, with aging variation of the base can be cancelled, and uniform toner densities can be obtained.DENS(toner patch)=R(toner patch)÷R(base one-circumference average)  (Equation 01)
Further, there has been a base correction method that realizes more accurate density measurement as compared to the above described base correction method.
FIG. 13A is a diagram showing an exemplary sensor output profile obtained by the density sensor reading the base over one circumference thereof. In the figure, the horizontal axis represents base phase (position), and the vertical axis represents sensor output. As will be understood from FIG. 13A, the surface gloss of the base not only varies over time, but also originally has unevenness within one circumference. The inconsistency affects sensor output values when toner patches are read.
FIG. 13B is a diagram showing a state in which a sensor output profile obtained from the base over one circumference thereof and a sensor output profile obtained with a toner patch put on the base are superposed in phase with each other. As will be understood from FIG. 13B, unevenness in the surface gloss of the base manifests itself in sensor output values of a highlight toner patch. Unevenness in the surface gloss of the base does not manifest itself in sensor output values of a toner patch whose density is so high that toner covers the base, but tends to manifest itself in sensor output values of a highlight toner patch because the surface of the base is partially exposed from the patch.
By canceling the above described effects which unevenness in the surface gloss of the base has on sensor output values, higher accurate density measurement can be realized as compared to the above described base correction method using the average sensor output value over one circumference as expressed by (Equation 01).
According to Japanese Laid-Open Patent Publication (Kokai) No. 2004-117807, when densities of toner patches are to be measured, one circumference of the base is read in advance using the density sensor, and phases of the base and sensor output values in the respective phases are stored. Then, by referring to the stored phases and sensor output values in the respective phases during density measurement, the densities of the toner patches are obtained by performing base correction with sensor output values of the base directly below the toner patches grasped.
Base correction with sensor output values of the base directly below toner patches grasped (density measurement) is performed using a method described hereafter. When there are a regular reflected light output R (toner patch) when a toner patch is read and a regular reflected light output R (base directly below toner patch) when the base directly below the toner patch is read, a density DENS (toner patch) of the toner patch is calculated according to (Equation 02) given below.DENS(toner patch)=R(toner patch)÷R(base directly below toner patch)  (Equation 02)
The toner patch density obtained by the above (Equation 2) is a density obtained with consideration given to the above described aging variations and unevenness within one circumference with respect to the base. Accurate density measurement can be realized using the above described base correction method.
To perform the above described base correction method considering aging variations of the base and unevenness within one circumference of the base, it is absolutely necessary to detect a phase of the base directly below a toner patch. To detect a phase of the base directly below a toner patch, it is necessary to rotate the intermediate transfer unit before forming the toner patch. For example, there has been a base correction method that rotates an intermediate transfer unit before forming a toner patch, measures a base profile for one circumference and base phases using a density sensor and a timer, forms a toner patch while continuing rotating the intermediate transfer unit, and measures the density of the toner patch.
However, the method that rotates the intermediate transfer unit before forming a toner patch, and measures a base profile for one circumference and base phases using the density sensor has the problem that the time period required for image density control increases because it is necessary to rotate the intermediate transfer unit before forming a toner patch.